Piston-mounted generator enhances engine tests

Columbus, IN--Ideally, most engine manufacturers would like to wire up their engines, install them in vehicles, then record detailed data on them for hundreds of hours.

Until now, however, gathering data in this way was nearly impossible. Accurate measurements of temperature, pressure, or strain inside a cylinder could only be obtained in a test cell, if they could be obtained at all.

Now that's changing. Two engineers from Cummins Engine Co., Inc., have developed a piston-mounted power generator that enables engineers to take measurements inside the engine as never before. Known as the Piston-Mounted Linear Power Generator, it provides electrical power for a miniature telemetry transmitter mounted on the piston. As a result, the device allows field testing of engines over periods of hundreds of hours. "This system opens up significant new possibilities for test engineers," notes Steven Archuleta, a Cummins technical specialist and co-inventor of the device.

Key to the system is a small, semi-circular generator module that mounts on the bottom of a piston. The generator, which powers a multi-channel telemetry system, is critical because it allows data acquisition without the need for wires leading in and out of the piston. As a result, the engine uses no so-called "grasshopper linkage," which can alter its performance characteristics during test.

The Piston-Mounted Linear Generator consists of magnets, pole pieces, an electrical coil, rectifier, and regulator. All of the parts are held in place in the generator module by an epoxy-based potting material.

The generator module works in conjunction with a specially designed cylinder liner. Like all conventional liners, it's made from ferromagnetic cast iron material. But unlike conventional liners, it has special grooves in it, into which engineers place a non-ferromagnetic material.

During operation, the magnets in the generator module rapidly pass by the interfaces between the ferromagnetic material and the non-ferromagnetic material. As the generator pole pieces align with the ferromagnetic material of the liner, a flux coupling, or magnetic circuit, forms between the magnets and the liner. Then, as the pole pieces pass the non-ferromagnetic grooves, the flux collapses. "The non-ferromagnetic grooves act like an air gap," explains Richard Varo, manager of high-horsepower applied mechanics at Cummins and co-inventor of the system. "So you get a rapid change in flux as the generator is swept past the grooves."

This rapid flux change induces a voltage in the generator's coil. The voltage passes through a conventional bridge rectifier, then drives the telemetry module. Output from the generator also powers sensors--thermocouples, pressure devices, strain gages, displacement transducers, and others.

Sensor outputs pass through a multiplexer, a voltage-controlled audio oscillator, and, finally, a frequency-modulated radio oscillator. An antenna mounted in the engine's oil pan gathers the FM signals for a receiver and demodulator located outside the engine. The transmitted signal has a frequency that is a function of the temperature or other parameter measured.

By operating in this way, the system harnesses the abundant mechanical energy already available within the engine and converts it to electrical energy. Used in conjunction with a conventional telemetry system, the generator module provides a data-acquisition method that produces results unavailable through other techniques. It does not suffer from the temperature and duration limitations associated with battery-powered methods. Nor does it require the extensive engine modifications of inductively powered systems. And unlike inertial generators, it can operate at low idle speed, and does not have high minimum speed requirements.

More important, the system requires no special linkages. That means that it does not upset the dynamics of the engine and it can be easily adapted for use in a vehicle.

"We wanted a system that could provide laboratory and field measurements for extended periods," Varo says. "This system does it without requiring any major modifications on the engine."

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